Data Set Overview
==================

This dataset contains 29 ionospheric electron density profiles
(EDS files) derived from Venera 15 and 16 radio occultation data.
A total of 155 profiles are believed to be available and it is
hoped that all will eventually be archived. These profiles were
produced by A. L. Gavrik, who worked on the Venera 15 and 16 radio
occultation experiment, and prepared for the PDS by P. Withers.

The DATA/ directory contains one .TAB file and one .LBL file for each
occultation. Each .TAB file contains a series of altitudes and
electron densities for an occultation. The LBL file contains
information about how to read the data and some ancillary information
about the data acquisition (time, location, etc.).

Each .TAB file in the ANC/ directory obeys the following naming
convention:

ANCYYYYMMDDHXXXQV.TAB

where YYYY is the year of the occultation (e.g. 1983), MM is the
month (e.g. 10), DD is the day of the month (e.g. 12), H is a
letter corresponding to the hour of the occultation, as discussed
in the next sentence, XXX is either V15 for Venera 15 occultations
or V16 for Venera 16 occultations, Q is either N for ingress or X for
egress, and V is a version label (e.g. A for the first version). An
occultation time of 05:54:00 has hour 05 and H is 'F'. Similiarly, an hour
of 00 leads to H being A, 01 to B, 02 to C, and so on until 23 to X. Thus
filename ANC19831012FV15NA.TAB corresponds to an ingress occultation on
1983-10-12 at a time with hour 05 by Venera 15, first version. Each
.LBL file in the ANC/ directory is named in a corresponding manner.
The same naming convention is used in the EDS/ directory, with the
replacement of 'ANC' in the filename with 'EDS'.

Dates and times used to produce these filenames are in the Moscow
winter timezone, not UTC. This is discussed further in the section
headed 'Timing'.

Parameters
==========

Each .TAB file in the ANC/ directory contains the following items.

Name of spacecraft (V15 or V16)                  SCT
Date of occultation, Moscow winter time          MOSCOW_WINTER_DATE
Time of occultation, Moscow winter time          MOSCOW_WINTER_TIME
Latitude of occultation, degrees north           LATITUDE
Longitude of occultation, degrees west           WEST_LONGITUDE
Solar zenith angle of occultation, degrees       SOLAR_ZENITH_ANGLE
Name of original file supplied by A. L. Gavrik   ORIG_FILENAME

The name of the original file supplied by A. L. Gavrik may contain
useful information. These files appear to follow some naming convention,
but this convention has not yet been fully understood. For instance,
information on the occultation summarized in file ANC19831012FV15A.TAB
was originally delivered in a file named 0566_b15.012.

Each .TAB file in the EDS/ directory contains the following items.

Altitude, km                                     ALTITUDE
Electron density, number per cubic centimeter    ELECTRON_DENSITY

The altitude is presumed to be relative to a sphere of radius 6052 km.

Uncertainties in these parameters are not specified precisely.
Uncertainty in absolute altitude is less than 2 km. Relative
altitudes for a given occultation are significantly more accurate.
Uncertainty in electron density varies with altitude, being smaller
at high altitudes than at low altitudes as a result of the data
processing techniques used. Hence the smallest electron densities
reported at low altitudes are significantly smaller than the
smallest reported at high altitudes. Uncertainty in electron density
at high altitude varies between 200 and 5000 electrons per cubic
centimeter, with a typical value of 500 electrons per cubic centimeter.
This value is sensitive to fluctuations in the total electron content
(i.e. electrons per unit area) along the ray of the radio signal.
Hence it is influenced by properties of the solar wind along the line
of sight at the time of the occultation. Uncertainty in electron
density near the ionospheric peak is approximately 10000 to 20000
electrons per cubic centimeter.

Timing
======

All dates and times reported in this dataset are in the Moscow winter
timezone, not UTC. This time zone is currently three hours ahead of UTC.
Times have not been converted into UTC in order to (A) eliminate the
possibility of introducing an error and (B) retain flexibility in the
case that delivery of additional profiles from the Venera 15 and 16
radio occultation experiments involves Moscow summer time.

Data Processing
===============

Data processing is described in the documents listed in the REF.CAT
file and in some of the other files in the CATALOG/ directory. The
principal equations used were:

N(p) = (-2 m f c) / (v-perp e^2) x
          integral from r=p to r=infinity of df dr / sqrt(r^2 - p^2)

and p = H + L c df / (v-perp f)

where N is electron density, p is the impact parameter of the radio
ray, m is the electron mass, f is the frequency, c is the speed of
light, v-perp is the line-of-sight component of the velocity of the
transmitter with respect to the receiver, e is the elementary charge,
df is the residual frequency change caused by the ionosphere of
Venus, H is the distance of closest approach of the ray to Venus, and
L is the separation between the spacecraft and Venus. These equations
are in Gaussian units, inserting 4 pi eps-0, where eps-0 is the
permittivity of free space, to the numerator of the right hand side
of the first equation converts them into SI units.

Data Coverage and Quality
=========================

Given the small number of available occultations, data coverage is
limited. No gaps have been identified in individual profiles.
Uncertainties in the ionospheric electron densities are not known
precisely, as discussed above.

Limitations and Caveats
=======================

These data products come from two Soviet spacecraft whose operations
ended over 25 years ago. Therefore the documentation and other aspects
of the dataset are not as complete as would be expected for a current
NASA mission. Nevertheless, it is felt that archiving the dataset in
its current state, thereby preserving it for posterity, is better
than risking that it be lost irrevocably.